Having said above "Ve'ahavta es Hashem Elokecha", why does the Torah see fit to add "es Hashem Elokecha Tiyra"?
Ramban #1: To deter a person from sinning and getting punished.
Ramban #2: Refer to 6:5:3:2*.
Ramban #3 (citing Temurah, 3b): It is a warning against uttering the Name of Hashem in vain.
Temurah, 4a: It is a warning against uttering the Name of Hashem in vain. 3
What are the connotations of "Oso Sa'avod"?
Ramban #1: It means that one is obligated to serve Hashem like an Eved serves his master.
Ramban #2: It means to serve Hashem by bringing Korbanos 1 in His Name, and by studying His Torah. 2
Ramban #3 (citing the Ibn Ezra): "Es Hashem Elokecha Tiyra" implies not to contravene Mitzvos Lo Sa'aseh, and "Oso Sa'avod", to keeping the Mitzvos Asei. 3
Rashi (in Eikev, 13:5) and Ramban (citing the Sifri) "ve'Oso Sa'avodu" - 'in the Beis-Hamikdash' (Ramban: incorporating Korbanos, singing and bowing, which all fall under the category of Avodah - as the Torah indicates in Bamidbar 18:7, 23').
Ramban, citing the same Sifri.
See Ramban's objection to this explanation.
Since when is it a Mitzvah to swear by Hashem's Name?
Rashi and Ramban #1 (citing the Tanchuma): What the Pasuk means is 1 that if one fears Hashem 2 and serves Him 3 he is permitted to swear by His Name; otherwise, not.
Ramban #2: The Pasuk means that one may only swear by the Name of Hashem, but not by the Name of any other god. 4
Ramban #3 (citing Temurah, 3b), Rosh and Targum Yonasan: The Pasuk is coming to permit swearing by Hashem's Name truthfully. 5
Ramban #4: What the Pasuk means is that Your Yir'as Shamayim will be so deep and you will be dedicated to His service 6 to such an extent, that you will always swear by His Name - even if it is to do something unpleasant - and abide by your oath.
Even though the two preceding statements in the Pasuk are Mitzvos - as the Ramban explains.
Ramban and Rosh: Like Avraham, Yosef and Iyov, who are called 'Yir'ei Elokim'.
Ramban: Someone whose only Avodah is that of Torah and Mitzvos.
Even if it is not for a Mitzvah - See Ramban. See also Peirush Yonasan and Torah Temimah, note 70, who elaborates.
Ramban: To the point that every mundane act that you perform is to serve Hashem - See Ramban DH 'O Yomar'.
Why does the Torah add "u'Bo Sidbak" in Eikev 10:20 and omit it here?
Ba'al ha'Turim: Because there, the Torah mentioned earlier Hashem's love for the Avos, which obligates their descendants to cling to Him.
Panim Yafos #1: Because, whereas in Eikev the Torah is discussing a Shevu'ah that one makes not for a Mitzvah - such as to avoid paying a debt, which is subject to "Bo Sidbak", here it discussing swearing in order to fulfill Mitzvos, which it permits ynconditionally.
Panim Yafos #2: Because, whereas here Moshe is referring to the Dor ha'Midbar all of whom were Chachamim who clung to Hashem 1 -in Eikev he is referring to the future generations after they enter Eretz Yisrael, who may only wear if they are Davuk to ashem.
Ha'amek Davar: Because, whereas here Moshe is referring to the soldiers who go to war, all of whom are fulfilling "Oso Sa'avod", in which case "Bo Sidbak" was not necessary. Moreover, it is impossible to fulfill "Bo Sidbak" on the battlefield; in Eikev, with reference to the people at large, he is telling them that by clinging to Hashem (marrying one's daughter to a Talmid-Chacham), they will merit to attain the level of "Oso Sa'avod".
Chizkuni (in 10:20): Because whereas in Eikev, Moshe said "u'Vo Sidbak" only after listing some of Hashem's Midos, 2 here where he did not mention any Midos, he omitted, "u'Vo Sidbak".
